1. Field of the Invention
The present invention relates to a saddle riding type vehicle mainly used for travelling on rough lands, and more particularly to a vehicle of the type wherein a pair of left and right driving wheels driven by a power unit are suspended at the front or rear portion of a vehicle body, each wheel having an extremely low pressure wide tire mounted thereon, and a saddle and a step are disposed at an intermediate and upper portion and a lower portion, respectively, of the vehicle body.
2. Description of the Prior Art
Generally, the vehicle of this kind is provided with a relatively lengthy step, and an operator or a driver operates the vehicle while moving his weight in all directions according to the condition of the rough land and to the running condition. It is often that one driving wheel is levitated from the ground during such running operation.
The vehicle of this kind has two types. One is that both the right and left driving wheels are integrally connected through a common axle driven by a power unit, and the other is that both the driving wheels are connected through a differential device.
In the former type, even if one of the right and left driving wheels is disengaged from the ground, the other driving wheel in the state in contact with the ground will not lose its driving force, and therefore, it has an excellent running property. On the other hand, when the vehicle is in a normal turning with both the driving wheels placed in contact with the ground, it is not possible to impart a rotational difference to both the driving wheels, thus producing the following inconveniences. That is, particularly, the tire of each driving wheel is of the extremely low pressure type, and when in normal turning operation, if a load imposed on each tire is varied under the influence of a centrifugal force acting on the vehicle body, the tire of the outer wheel at which applied load increases is compressed to suddenly reduce its radius in contact with the ground whereas the tire of the inner wheel inflates to suddenly increase the radius in contact with the ground. For this reason, a great difference occurs in the effective radius between both the driving wheels and in addition a difference in length of turning orbit between both the inner and outer wheels occurs, resulting in a severe slip between the inner wheel having a low ground contact force and the ground. As a result, the tire of the inner wheel may damage the lawn over a wide area and destroy the ridges of a farm.
In the latter type, it is possible to impart relative rotation to both the driving wheels even in the normal turning operation so that the vehicle can be operated without doing damage to the ground. However, when one of the driving wheels is disengaged from the ground, the other driving wheel in the state in contact with the ground will lose its driving force, deteriorating the running property.
In order to solve these problems, a proposal has been made in which a differential device is improved in such a manner that when a difference of rotational speed between both driving wheels occurs, a power from a power unit to the driving wheel on the high speed side is cut off. (See Japanese Patent Application Laid-Open No. 136563/83.)
According to such improved type of differential device as described above, the supply and cutoff of power to the high speed-side driving wheel to be effected according to the difference of rotational speed between both the driving wheels causes shocks to impair riding comfort. In addition, at the time of turning, only the inner side driving wheel is driven, and so a sufficient running property is not obtained therefrom.